Measuring devices, which are configured for progressive tracking of a target point and a coordinate position determination of this point, can generally be summarized under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, a cube prism), which is targeted using an optical measuring beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is captured using a capture unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the capture of the beam, for example, by means of runtime or phase difference measurement or by means of the Fizeau principle and—increasingly as a standard feature in modern systems—an offset of the received beam from a zero position is ascertained on a sensor.
By means of this offset which is thus measurable, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected and/or tracked as a function of this deviation such that the offset on the sensor is reduced, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking (tracking) of the target point can be performed and the distance and position of the target point can be progressively determined in relation to the measuring device. The tracking can be implemented in this case by means of an alignment change of the deflection mirror, which is movable by a motor and is provided for deflecting the laser beam, and/or by pivoting the targeting unit, which has the beam-guiding laser optical unit.
Laser trackers according to the prior art can additionally be embodied having an optical image capture unit, in particular having a two-dimensional, light-sensitive sensor having an image processing unit, in particular having a camera, the optical unit of which is arranged separately from the optical unit of the laser beam. Using the capture and analysis of an image—by means of image capture unit and image processing unit—of a so-called measuring aid instrument having markings, the relative location of which in relation to one another is known, an orientation of an object (for example, a probe), which is arranged on the measuring aid instrument, in space can be concluded. Together with the determined spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or in relation to the laser tracker.
Such laser trackers and methods for the six degrees of freedom determination, in particular by means of corresponding measuring aid instruments, are described, for example, in European patent application 14179139.2.
Laser trackers of the prior art at least have a distance meter for distance measurement, wherein it can be configured, for example, as an interferometer. Because such distance measuring units can only measure relative distance changes, so-called absolute distance meters are installed in current laser trackers in addition to interferometers. For example, such a combination of measuring means for distance determination is described in WO 2007/079600 A1.
The interferometers used in this context for the distance measurement primarily use—because of the long coherence length and the measurement range thus enabled—HeNe gas lasers as light sources. The coherence length of the HeNe laser can be several hundred meters in this case, so that the ranges required in industrial metrology can be achieved using relatively simple interferometer structures. A combination of an absolute distance meter and an interferometer for distance determination using a HeNe laser is known, for example, from WO 2007/079600 A1.
The use of HeNe laser light sources has the disadvantage, however, with regard to a generally desirable miniaturization of laser trackers, of the size thereof, which determines the light power. The power of the light source is significantly dependent in this case on the length of the laser tubes, i.e., the longer the tubes, the greater the achievable emission power. In addition, such a laser source typically displays a relatively high level of power dissipation. The high voltage supply required for operation represents a further disadvantage. For example, a voltage of approximately 7000 V has to be provided for the ignition of the laser and a voltage of approximately 1500 V has to be provided during operation, whereby special components (for example, a high-voltage power supply unit and shield) have to be used and safety measures have to be taken upon the use of such light sources. The sensitivity in relation to magnetic fields (for example, generated by internal motors or external welding transformers) and the limited service life of the tubes (typically approximately 15,000 operating hours) also make the use of HeNe lasers disadvantageous, for example, because the light sources often have to be replaced in the systems in a costly manner.
In principle, HeNe lasers can be replaced by diode lasers as the light source for the interferometer. These laser diodes are compact per se, cost-effective, and have a low power consumption. The following laser diode sources are often used in particular for the use as an interferometer light source:                distributed feedback laser (DFB) (having a periodically structured active medium, for example, lattice),        distributed Bragg reflector laser (DBR) (having an optical lattice outside the active medium but arranged on a shared chip),        fiber Bragg grating laser (FBG) (essentially according to a DFB laser, but having a lattice in an external fiber),        external cavity diode laser (ECDL) (stabilization of the laser diode by means of an external highly stable cavity, for example, having a holographic lattice),        diode pumped solid-state lasers (DPSS),        discrete mode lasers (DMD),        microchip lasers, and/or        surface emitter lasers (VCSEL).        
The beam sources are configured in this case such that the emitted laser beam, with respect to the wavelength, is single mode having a coherence length in the order of magnitude of several tens of meters (and/or a line width <1 MHz).
In addition, a stabilization at a known wavelength is necessary for the use of such laser diodes as an interferometer light source or as a wavelength standard. This can be performed, for example, spectroscopically on an absorption line of an absorption medium (for example, using a gas cell). In this case, a very large number of absorption lines can occur in a desired wavelength range depending on the absorption medium used. On the one hand, so many absorption lines are present that even in the event of manufacturing-related scattering of the emission wavelength of the laser diode, an absorption line is always achievable for stabilization, on the other hand, this line also has to be unambiguously identified upon each restart of the light source to establish the emission wavelength.
For this purpose, in principle it can be stabilized simply on any suitable and defined line and this can be identified in production using an external wavelength meter. By means of storage and reproduction of the diode parameters set for this purpose, for example, temperature and current, with perfect control electronics, one should again land on the original line and find it again using a short wavelength scan. A possible change of the setting parameters of the diode due to aging can be resisted by storing the respective last values.
The requirements for the measuring device are similarly transferable to measuring devices which have an interferometer unit for determining distance changes. In this case, measuring devices which are configured for progressive tracking of a target point and a coordinate position determination of this point can generally be summarized under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, a cube prism) which is targeted using an optical measuring beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is captured using a capture unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the capture of the beam, for example, by means of runtime or phase difference measurement.
A use of a laser diode as an interferometer laser light source is described, for example, in European patent application EP 2 589 982. European patent applications EP 2 662 661 and EP 2 662 702 describe further tunable laser diodes for stabilizing the emission wavelength for use in a laser tracker, in particular wherein an unambiguous identification of an absorption line used for stabilization can be carried out, in particular upon each restart of the system. Finally, European patent application 14179139.2 describes a complete system for a coordinate measuring device of the type in question for measuring coordinates on surfaces of target objects, in particular embodied having a laser tracker having a laser diode beam source.
The use of a specific laser diode with the interferometer of the laser tracker offers advantages with respect to the space requirement linked thereto, which results as significantly less than a HeNe gas laser with greater coherence length at the same time, however. In contrast to a gas laser source, which can also provide measuring radiation having suitable coherence length, in addition, a high-voltage supply is not required for operating the diode. Furthermore, such laser diodes have a lower power consumption.
Laser tracker systems of the type in question having laser diodes according to the prior art furthermore typically have a specific control of the laser diode, in the context of which the laser trackers and the control unit of the laser diode are configured such that the emission wavelength of the measuring radiation is longitudinally variable in a monomodal manner within a specific emission wavelength range. In this case, the emission wavelength can be variable by a temperature change of the laser diode and/or a change of an electrical current applied to the laser diode, controlled by the control unit. Furthermore, the laser diode can be controllable by means of the control unit such that an emission power of the measuring radiation is variable.
A typical construction of a laser diode beam source in a free beam setup according to the prior art typically comprises a laser diode having collimation optical unit, a free beam isolator, and a coupling into an optical fiber, wherein typically the diode and the collimation optical unit are located in a thermo-electrically temperature-stabilized cell (TEC cell). Upon the use of such a setup in a laser tracker, however, it is particularly disadvantageous that as a result of the large operating temperature scope in the laser tracker, the pointing stability requires an active control of the beam direction to keep the coupling efficiency into the fiber. In addition to the complex control and calibration of the laser beam source linked thereto, a further optimization of the space required in the laser tracker is additionally limited. More extensive miniaturization of such a laser diode beam source is therefore only implementable under laboratory conditions.